1) Field of the Invention
The present invention relates to a method and apparatus for handing off a mobile station from a wireless base station to another wireless base station in a mobile communication network.
2) Description of the Related Art
In the field of mobile communications, there are demands for high quality service. In particular, in the current mobile communications, the quality of service is often lowered during operations for achieving a handoff as described below.
The handoff is an operation of transferring control of a mobile station from a first wireless base station to a second wireless base station when the mobile station moves from an area covered by the first wireless base station to another area covered by the second wireless base station. The operations for achieving a handoff is performed based on comparison of at least one reference signal received from at least one wireless base station with a given handoff threshold.
FIG. 8 is a flow diagram illustrating an outline of a typical one of the conventional operations for achieving a handoff in the conventional mobile communication networks.
In step S40, the mobile station receives a predetermined handoff threshold from a wireless base station currently controlling the mobile station, which is hereinafter called a currently controlling wireless base station.
In step S41, the mobile station constantly searches for a reference signal from each of at least one other wireless base station.
In step S42, the mobile station compares the monitored reference signal with the above handoff threshold. When the level of the monitored reference signal exceeds the above handoff threshold, the operation goes to step S43. When the level of the monitored reference signal does not exceed the above handoff threshold, the operation goes to step S41. Conventionally, the hand-off threshold is a constant.
In step S43, the mobile station sends a report signal to the currently controlling wireless base station, where the report signal indicates that there is a candidate for a substitute wireless base station to which control of the mobile station is to be handed off.
In step S44, when the currently controlling wireless base station receives the report signal from the mobile station, and confirms validity of the contents of the report signal, the currently controlling wireless base station instructs the candidate for the substitute wireless base station to secure resources, such as a traffic channel, for use in control of the mobile station.
In step S45, the currently controlling wireless base station sends to the mobile station an over-the-air message through a traffic channel or a control channel, where the over-the-air message instructs the mobile station to hand off the control to the substitute wireless base station, and includes information such as frequencies and spreading codes to be used between the mobile station and the substitute wireless base station.
In step S46, when the mobile station receives the over-the-air message, the mobile station switches a communication channel from the currently controlling wireless base station to the substitute wireless base station.
In step S47, a communication path is established between the mobile station and the substitute wireless base station. Thus, the operation for achieving a handoff is completed.
However, the conventional handoffs as described above have the following drawbacks.
FIG. 9 is a diagram illustrating an example of a situation in which the handoff is liable to fail. In the situation of FIG. 9, the wireless base stations 100 and 200 are arranged in the areas C1 and C2, respectively. The mobile station MS is currently connected to the wireless base station 100, and is moving toward the wireless base station 200. However, if the handoff threshold so high that the level of the reference signal from the wireless base station 200 exceeds the handoff threshold in the vicinity of the boundary of the area C1, it is probable that quality (quality of the wireless link, received field strength, and the like) of a wireless signal received from the currently controlling wireless base station to the mobile station MS deteriorates. Therefore, the over-the-air message from the currently controlling wireless base station may not be successfully received by the mobile station, and thus the handoff may not succeed. In addition, when the moving speed of the mobile station is high, the probability of failure in the handoff is further increased.
In some regions in the service area of a mobile communication network, a complex boundary may be formed between different areas covered by different wireless base stations, due to fading or shadowing. When a mobile station moves in such regions, an unnecessarily great number of handoffs may be performed.
FIG. 10 is a diagram illustrating an example of a situation in which a mobile station MS moves through a region in which a complex boundary is formed between different areas C1′ and C2′ respectively covered by different wireless base stations 100′ and 200′. As illustrated by arrows in FIG. 10, the mobile station MS crosses the complex boundary four times. In this situation, the levels L1 of the reference signal from the wireless base station 100′at the points P1, P3, and P5 are greater than the handoff threshold TH, and the levels L2 of the reference signal from the wireless base station 200′ at the points P2 and P4 are also greater than the handoff threshold TH. Therefore, when the mobile station MS moves through the path indicated by the arrows in FIG. 10, the handoffs are performed unnecessarily frequently.